Electromagnetic Stirrer Coil

ABSTRACT

The present invention provides a previously unattainable compact and high thrust electromagnetic stirrer coil, that is, an electromagnetic stirrer coil for stirring molten steel in a mold by electromagnetic force, in which electromagnetic stirrer coil a space factor of the yoke sectional area (−) with respect to an inside area in a horizontal cross-section of said electromagnetic stirrer coil is 0.5 to 0.9 and a yoke width B is 100 mm to 300 mm. Preferably, a magnetomotive force F of said electromagnetic stirrer coil divided by the yoke width B, that is, a value of F/B, is 800 kAT/m 2  or more.

TECHNICAL FIELD

The present invention relates to an electromagnetic stirrer coil forstirring molten steel in a mold by electromagnetic force.

BACKGROUND ART

In the past, in a continuous casting facility, to cause nonmetallicinclusions included in the molten steel in a mold and bubbles of Ar gasblown into an immersion nozzle to rise to the surface of the moltensteel without being trapped in the slab and thereby obtain a goodquality slab, the method has been used of stirring the molten steel inthe mold by electromagnetic force. Various proposals have been made inthe past relating to electromagnetic stirrer coils for stirring moltensteel in a mold by electromagnetic force.

For example, Japanese Patent No. 3273105 discloses a fluid motioncontrol system providing a second core abutting against a back surfaceof a first core (yoke) having slots for winding of a coil and a thirdcore abutting against the top and bottom surfaces of the first core(yoke) so as to increase the effective area of the core and increase thesaturation flux density and thereby enable a stronger magnetic field tobe applied to the molten metal while retaining about the same outsideshape as in conventional systems.

However, Japanese Patent No. 3273105 discloses a method of increasingthe effective area of the core (yoke), but the specific ranges ofnumerical values of the space factor of the yoke sectional area (−) withrespect to the inside area in the horizontal cross-section of theelectromagnetic stirrer coil corresponding to that effective area andthe yoke width B were not sufficiently studied, so a compact and highthrust electromagnetic stirrer coil could not be realized.

DISCLOSURE OF THE INVENTION

The present invention has as its object to solve the above problems inthe prior art and provide a never previously attainable compact and highthrust electromagnetic stirrer coil.

The inventors engaged in in-depth studies to achieve the above objectand as a result provided a compact and high thrust electromagneticstirrer coil by specifying preferable ranges of numerical values for thespace factor of the yoke sectional area (−) with respect to an insidearea in a horizontal cross-section of the electromagnetic stirrer coilcorresponding to the effective area of the core (yoke) and for the yokewidth B. It has as its gist the following content:

(1) An electromagnetic stirrer coil for stirring molten steel in a moldby electromagnetic force, said electromagnetic stirrer coilcharacterized in that a space factor of the yoke sectional area (−) withrespect to an inside area in a horizontal cross-section of saidelectromagnetic stirrer coil is 0.5 or more and a yoke width B is 100 mmto 300 mm.

(2) An electromagnetic stirrer coil as set forth in (1) characterized inthat a magnetomotive force F of said electromagnetic stirrer coildivided by the yoke width B, that is, a value of F/B, is 800 kAT/m ormore.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 are views illustrating an embodiment of an electromagneticstirrer coil in the present invention, wherein (a) is a plan view and(b) is a side view.

FIG. 2 is a detailed view (sectional view) of the top of a moldincluding the electromagnetic stirrer coil in the present invention asseen from the side surface.

FIG. 3 is a detailed view of an electromagnetic stirrer coil part in thepresent invention.

FIG. 4 is a view showing the relationship between the yoke width B andthe above-mentioned space factor.

FIG. 5 is a view showing the relationship between the space factor (−)and the magnetomotive force for obtaining the necessary thrust.

FIG. 6 is a view showing the relationship between the yoke width B andthe magnetomotive force F/yoke width B.

FIG. 7 is a view showing the results of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the present invention will be explainedin detail using FIG. 1 to FIG. 7.

FIG. 1, FIG. 2, and FIG. 3 are views illustrating an embodiment of anelectromagnetic stirrer coil in the present invention.

In FIG. 1 and FIG. 2, 1 indicates a mold, 2 an electromagnetic stirrercoil, 3 an immersion nozzle, 4 molten steel, 5 a strand pool, and 6 ayoke.

FIG. 1(a) is a plan view of the electromagnetic stirrer coil of thepresent invention, while (b) is its side view.

The mold 1 of a continuous casting machine is filled with molten steel4. By running a current through the electromagnetic stirrer coil 2arranged around that mold 1, an electromagnetic force is generated,thrust in the arrow (solid line) direction acts on the molten steel 1,and the molten steel 4 in the strand pool 5 is stirred.

Further, at the center of the strand pool 5, the immersion nozzle 3 isset. This immersion nozzle 3 injects molten steel into the mold. As aresult, a flow of molten steel 4 (broken line) is formed. Formation ofthese two flows without allowing any interference between them isnecessary for casting a good quality slab.

FIG. 2 is a detailed view of the mold part including the electromagneticstirrer coil in the present invention as seen from the side surface(horizontal cross-section), while FIG. 3 is an enlarged view (sectionalview) of the coil part.

Inside the electromagnetic stirrer coil 2 is placed the yoke 6corresponding to a core. Power is supplied to the coil wound around thisyoke to generate a magnetic field. The present invention ischaracterized in that the space factor (−) of the sectional area (B×D)of the yoke 6 with respect to the inside area in the horizontalcross-section of the electromagnetic stirrer coil 2 (specifically theinside area surrounded by the outside shape 7 of the coil window of FIG.3) is 0.5 or more and the yoke width B is 100 mm to 300 mm.

First, the reasons for limitation of the yoke width B will be explained.

The yoke width B in the horizontal cross-section of the electromagneticstirrer coil 2 shown in FIG. 2 is made 100 mm or more because 100 mm ormore is necessary in order to try to improve the cleanliness of the slabsurface part by imparting fluid motion to the front surface of thesolidified shell.

Further, the yoke width B in the horizontal cross-section of theelectromagnetic stirrer coil 2 is made 300 mm or less becauseinterference between the flow discharged from the nozzle and the stirredflow can be avoided and a swirl can be stably formed near the meltsurface. It is preferable to make the yoke width B smaller than theimmersion depth L shown in FIG. 2. In general, the immersion depth L is300 mm or so, therefore the upper limit was made 300 mm. Further,preferably, if the yoke width B is 250 mm or less, it is possible toreliably avoid interference between the flow discharged from the nozzleand the stirred flow.

Next, the reason for making the space factor (−) of the yoke 0.5 or morewill be explained.

The inside area in the horizontal cross-section of the electromagneticstirrer coil 2, more specifically the inside area surrounded by theoutside shape 7 of the coil window of FIG. 3, shows the size of theelectromagnetic stirrer coil 2. The smaller this inside area, the morecompact the electromagnetic stirrer coil becomes.

The magnitude of the magnetic force able to be formed by supplyingcurrent to the electromagnetic stirrer coil 2 is defined by themagnetomotive force. A high efficiency is realized if able to form themagnetic field able to be produced by that magnetomotive force insidethe yoke 6 without magnetic saturation. Once magnetically saturated,even if increasing the magnetomotive force of the electromagneticstirrer coil 2 over this, it is not possible to form a magnetic fieldcommensurate with the increase in the magnetomotive force.

On the other hand, the maximum value of the magnetomotive force is 200kAT/m or so. If over this, the problem of local heat buildup of the yoke6 arises and steps such as making the yoke 6 an internally water cooledstructure become necessary.

The inventors investigated the relationship between the space factor (−)of the sectional area (B×D) of the yoke 6 with respect to the insidearea in the horizontal cross-section of the electromagnetic stirrer coil2 and the obtained thrust under the condition of a yoke width of 100 to300 mm whereupon they learned that by making the space factor (−) 0.5 ormore, substantially the desired thrust is obtained.

Therefore, in the present invention, the space factor (−) of thesectional area (B×D) of the yoke 6 with respect to the inside area inthe horizontal cross-section of the electromagnetic stirrer coil 2(specifically, the inside area surrounded by the outer shape 7 of thecoil window of FIG. 3) was made 0.5 or more. (See FIG. 5.)

In the present invention, the upper limit of the space factor is notdefined, but from the viewpoint of the ease of production, 0.9 or lessis a preferable range.

Further, according to the present invention, if there is leeway in thepower capacity or if there is leeway in the flux density in the yoke toenable the magnetomotive force for obtaining the prescribed thrust to beobtained, it is also possible to increase the thrust in accordance withneed.

Note that in the present invention, the method of increasing the spacefactor is not critical, but it is preferable to reduce the outside shapeof the water cooled copper pipe forming the coil to for example 4.0 mmor less to reduce the bending radius of the copper pipe and therebybring the inside shape of the coil close to the sectional shape of theyoke.

Further, the magnetomotive force F of the electromagnetic stirrer coildivided by the yoke width B, that is, the value of F/B, is preferably800 kAT/m or more. This is because making the magnetomotive force F/yokewidth B 800 kAT/m or more avoids interference between the flowdischarged from the immersion nozzle and the stirred flow and enables astirring speed required for prevent inclusions from being trapped in thesolidified shell to be obtained.

EMBODIMENT

An embodiment of the electromagnetic stirrer coil of the presentinvention will be shown in FIG. 4 to FIG. 6.

The inventors prepared several coils differing in yoke width and spacefactor and investigated whether the prescribed thrust of 10,000 Pa/mcould be obtained. Here, the “thrust” means the value of the forceacting on a brass plate measured using a strain gauge etc. in the stateplacing the brass plate at a position 15 mm from the inside wall of themold and running current through the electromagnetic stirrer coil and isshown in units of Pa/m.

Further, the inventors used the electromagnetic stirrer coils for actualcasting. The type of the steel was low carbon Al killed steel. Thismolten steel was cast into a slab of a thickness of 250 mm and a widthof 1800 mm. The casting speed was 1 m/min. The nozzle was run throughwith Ar gas at a rate of 3 Nl/min. The immersion depth L was made 300mm. Regarding the number of bubbles and inclusions at the surface partof the slab, the inventors cut out samples of the total width×castingdirection length 200 mm from the top surface and bottom surface of theslab, ground away the bubbles and inclusions in a surface of the totalwidth×length 200 mm at every other 1 mm from the surface, andinvestigated the sum of the numbers of bubbles and inclusions of 100microns or more size down to 10 mm from the surface.

In addition, to clarify whether or not the stirred flow by theelectromagnetic stirrer coil and the flow discharged from the immersionnozzle will interfere with the flow rising along the short sides to nearthe melt surface inside the mold, the inventors investigated thesolidified structure in the horizontal cross-section of the slab.

FIG. 4 is a view showing the relationship between the yoke width B andthe above-mentioned space factor. In FIG. 4, the scope of the presentinvention is shown by the arrows. That is, when the preparedelectromagnetic stirrer coils had a space factor of 0.5 or more and acore thickness of 100 mm to 300 mm, the prescribed thrust stirring couldbe imparted. Further, under those conditions, even if investigating thesolidified structure of the slab, it was confirmed that the dendritesgrowing from the slab surface toward the inside grew with a uniformangle in the upwind direction of the flow across the slab total width.

FIG. 5 is a view of the relationship between the space factor (−) andthe magnetomotive force for obtaining a prescribed thrust. Note thatFIG. 5 includes several plots. These show the results of preparation ofseveral electromagnetic stirrer coils with different space factors andstudy of the conditions for giving the target thrust of 10,000 Pa/munder the respective conditions. From FIG. 5, by making the space factor(−) 0.5 or more, the required thrust can be applied without magneticsaturation. Here, the rapid increase in the magnetomotive force with aspace factor (−) of less than 0.5 shows that magnetic saturation hasoccurred.

The relationship between the magnetomotive force F/yoke width B and thedefects occurring in a slab when using the several electromagneticstirrer coils differing in yoke width B and magnetomotive force F/yokewidth shown in FIG. 6 is shown in FIG. 7. The “defect index” shown atthe ordinate of FIG. 7 shows the sum of the number of bubbles andinclusions down to 10 mm from the slab surface found under severalconditions and indexed to the number when not applying electromagneticstirring as “1”. In FIG. 7, it was confirmed that increasing themagnetomotive force/yoke width reduces the defect index, but inparticular making it 800 kAT/m or more enables remarkable reduction.Based on the results of FIG. 7, FIG. 6 shows the preferable range of thepresent invention by arrows.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide a compactand high thrust electromagnetic stirrer coil by specifying preferableranges of numerical values for the space factor of the yoke sectionalarea (−) with respect to an inside area in a horizontal cross-section ofthe electromagnetic stirrer coil corresponding to the effective area ofthe core (yoke) and for the yoke width B, interference between thestirred flow and the flow discharged from the immersion nozzle can beavoided and a swirl can be stably formed near the melt surface, andother useful remarkable effects in industry are exhibited.

1. An electromagnetic stirrer coil for stirring molten steel in a moldby electromagnetic force, said electromagnetic stirrer coilcharacterized in that a space factor of the yoke sectional area (−) withrespect to an inside area in a horizontal cross-section of saidelectromagnetic stirrer coil is 0.5 or more and a yoke width B is 100 mmto 300 mm.
 2. An electromagnetic stirrer coil as set forth in claim 1,characterized in that a magnetomotive force F of said electromagneticstirrer coil divided by the yoke width B, that is, a value of F/B, is800 kAT/m or more.